Typical grooved wicks, used in spacecraft constant conductance heat pipes (CCHPs), diodes and Variable Conductance Heat Pipes (VCHPs), have a very high permeability, allowing very long heat pipes for operation in zero-g, typically several meters long. However, axial grooved constant conductance heat pipes have a relatively low heat flux limitation, on the order of 5 W/cm2 before the heat pipe conductance drops off.
These grooved aluminum/ammonia heat pipes are designed to work with a 0.10 inch adverse elevation (evaporator elevated above the condenser). This allows them to be tested on earth prior to insertion in a spacecraft. However, they are very sensitive to adverse elevation. For example, increasing the heat pipe elevation by 0.10 inch will significantly decrease the power.
For applications with higher heat fluxes or with adverse elevation, loop heat pipes (LHPs) are currently used in place of constant conductance heat pipes. The disadvantage of LHPs is that they are significantly more expensive to fabricate and often are more difficult to start-up, sometimes requiring start-up heaters.
Heat spreaders are used to reduce the heat flux generated by the component to a level that is manageable by the heat pipe. Heat spreaders typically consist of aluminum plates but may also be made of carbon composites, pyrolytic graphite, copper, or any other thermally conductive material and/or other heat pipe assemblies. The use of heat spreaders can add significant weight, volume and cost the system. The thermal resistance of the system is also increased since at least two more thermal interfaces plus the conduction path of the spreader itself is introduced.
One type of heat spreader uses copper water heat pipes embedded into aluminum. Copper water heat pipes use a porous wick structure and are capable of handling heat fluxes up to 50 W/cm2. Heat pipe embedded aluminum plates are used as heat spreaders and in some cases also as a structural member in electronics packaging. Embedding heat pipes increases the effective thermal conductivity by several factors without negatively affecting the plate's mass, strength or corrosion resistance. When designed properly, they can also operate against adverse elevations. In general, the performance of a heat pipe embedded aluminum plate is better than that of the high end composite materials but costs much less to manufacture. The typical thermal conductivity is roughly 600 to 1200 W/cm2. The layout of the embedded heat pipes may be optimized based on the heat source profiles and locations. A higher number of heat pipes may be embedded in areas on the plate where large heat sources are attached. Even with the embedded heat pipes, the heat pipe embedded plate may weigh less than an equivalently sized conventional aluminum plate.
However, as the trend for electronics is driving toward higher performance from a smaller package, the heat flux is increased, which thereby increases the importance of thermal management. This is especially true for the satellite and aerospace industry where size and performance are critical design considerations. The use of heat spreaders is not sufficient to reduce the heat flux from the source to a level that can be accepted by the constant conductance heat pipe while allowing for reduced weight, volume, thermal resistance and cost to the system.
It would, therefore, be beneficial to provide a heat pipe assembly which can reduce the heat flux from the source to a level that can be accepted by the constant conductance heat pipes while also reducing the weight, volume, thermal resistance and cost to the system.